1. Field of the Invention
This invention relates generally to perpendicular magnetic recording (PMR) media, such as perpendicular magnetic recording disks for use in magnetic recording hard disk drives, and more particularly to a perpendicular magnetic recording disk with a granular cobalt-alloy recording layer having controlled grain size.
2. Description of the Related Art
In a PMR disk, the recording layer is a layer of granular cobalt-alloy magnetic material that becomes formed into concentric data tracks containing the magnetically recorded data bits when the write head writes on the magnetic material.
FIG. 1 is a schematic of a cross-section of a prior art PMR disk. The disk includes a disk substrate and an optional “soft” or relatively low-coercivity magnetically permeable underlayer (SUL). The SUL serves as a flux return path for the field from the write pole to the return pole of the recording head. The material for the recording layer (RL) is a granular ferromagnetic cobalt (Co) alloy, such as a CoPtCr alloy, with a hexagonal-close-packed (hcp) crystalline structure having the c-axis oriented substantially out-of-plane or perpendicular to the RL. The granular cobalt alloy RL should also have a well-isolated fine-grain structure to produce a high-coercivity (Hc) media and to reduce intergranular exchange coupling, which is responsible for high intrinsic media noise. Enhancement of grain segregation in the cobalt alloy RL is achieved by the addition of oxides, including oxides of Si, Ta, Ti, Nb, B, C, and W. These oxides (Ox) tend to precipitate to the grain boundaries as shown in FIG. 1, and together with the elements of the cobalt alloy form nonmagnetic intergranular material. An optional capping layer (CP), such as a granular Co alloy without added oxides or with smaller amounts of oxides than the RL, is typically deposited on the RL to mediate the intergranular coupling of the grains of the RL, and a protective overcoat (OC) such as a layer of amorphous diamond-like carbon is deposited on the CP.
The Co alloy RL has substantially out-of-plane or perpendicular magnetic anisotropy as a result of the c-axis of its hexagonal-close-pack (hcp) crystalline structure being induced to grow substantially perpendicular to the plane of the layer during deposition. To induce this growth of the hcp RL, intermediate layers of ruthenium (Ru1 and Ru2) are located below the RL. Ruthenium (Ru) and certain Ru alloys, such as RuCr, are nonmagnetic hcp materials that induce the growth of the RL. An optional seed layer (SL) may be formed on the SUL prior to deposition of Ru1.
The enhancement of segregation of the magnetic grains in the RL by the additive oxides as segregants is important for achieving high areal density and recording performance. The intergranular Ox segregant material not only decouples intergranular exchange but also exerts control on the size and distribution of the magnetic grains in the RL. Current disk fabrication methods achieve this segregated RL by growing the RL on the Ru2 layer that exhibits columnar growth of the Ru or Ru-alloy grains.
FIG. 2 is a transmission electron microscopy (TEM) image of a portion of the surface of a prior art CoPtCr—SiO2 RL from a disk similar to that shown in FIG. 1. FIG. 2 shows well-segregated CoPtCr magnetic grains separated by intergranular SiO2 (white areas). However, as is apparent from FIG. 2, there is a relatively wide variation in the size of the magnetic grains and thus the grain-to-grain distance. A large grain size distribution is undesirable because it results in a variation in magnetic recording properties across the disk and because some of the smaller grains can be thermally unstable, resulting in loss of data. FIG. 2 also illustrates the randomness of grain locations. Because the nucleation sites during the sputtering deposition are randomly distributed by nature, there is no control of the grain locations. The amount of Ox segregants inside the RL needs to be sufficient to provide adequate grain-to-grain separation, but not too high to destroy the thermal stability of the RL. The typical content of the Ox segregants is about 20% in volume, and the grain boundary thickness is typically between about 1.0 and 1.5 nm.
To achieve high areal density of 1 to 5 Terabits/in2 and beyond in PMR media, it is desirable to have high uniformity (or tighter distribution) of the grains within the RL, mainly for the following three structural parameters: grain diameter (i.e., the diameter of a circle that would have the same area as the grain), grain-to-grain distance (i.e., the distance between the centers of adjacent grains or “pitch”) and grain boundary thickness. Narrower distribution of these three structural parameters will lead to narrower distributions of magnetic exchange interaction and magnetic anisotropy strength, both of which are desirable. Thus the prior art RL shown in FIG. 2 is far from ideal. First, the grains have an irregular polygonal shape with a large size distribution. Second, the location of the grain centers is highly random, which means there is no short range or local ordering, i.e., no pattern within approximately 3-5 grain distances. Third, the thickness of the grain boundaries (the Ox segregants seen as white areas in FIG. 2) has an even wider distribution.
In one approach for making PMR disks, sometimes called templated growth, the layers up to but not including the magnetic recording layer and its underlayers (called the magnetic stack) are deposited on the substrate. One type of templated growth uses a seed layer that is then deposited and lithographically patterned and pre-etched (i.e., before the deposition of the magnetic stack) to form a topographic pattern of seed layer pedestals surrounded by seed layer spaces or trenches. The material of the magnetic stack is then deposited on the topographically patterned seed layer, with magnetic material growing on the seed layer pedestals and the nonmagnetic material growing on the seed layer trenches. Another type of templated growth uses two types of seed layer material. A first seed layer is deposited and then lithographically patterned and pre-etched (i.e., before the deposition of the magnetic stack) to form a patterned first seed layer. Then a separate second seed layer is deposited into the etched spaces where there is no first seed layer. The material of the magnetic stack is then deposited on the patterned seed layers, with magnetic material growing on the first seed layer and nonmagnetic material growing on the second seed layer. One example of this approach, as described in U.S. Pat. No. 7,776,388 B2 assigned to the same assignee as this application, has etched ruthenium (Ru) as the first seed layer and oxide spaces as the second seed layer. Magnetic CoPtCr material and nonmagnetic oxide material is then sputter deposited simultaneously, with the CoPtCr growing on the Ru seed layer and the oxide growing on the oxide seed layer.
The etching of the template layer causes re-deposition of the template layer material. Re-deposition occurs when the material being etched is displaced from its original site via physical bombardment or chemical reaction, but cannot be properly volatized and removed in full. The re-deposition causes undesired topography and poor surface texture, which adversely affects the desired size and spacing of the subsequently formed magnetic islands. Also, because the subsequently deposited magnetic material and oxide material will generally replicate the pattern of the template layer, if a topographically patterned template layer is used a planarization process may be required to assure that the completed disk has a generally smooth planar surface.
What is needed is a PMR perpendicular magnetic recording disk with a substantially planar patterned template layer that does not have the disadvantages caused by etching of the template layer material.